In the metal forming and welding arts, the initial manufacturing process, the after-manufacture treatment of the product, the encountering of and the magnitude of loads in use and the aging process lead to deterioration of load bearing strength in the product structure, whether unseen without destructive analysis or evidenced by catastrophic failure, such as by appearance of fractures or cracks.
Conventional welded products are made by employment of various welding art technological operation steps before and after the actual welding step in an attempt to improve the working life of the products. Some of these technological operation steps are categorized as: (a) pre-welding preparation of exposed surfaces at welding sites by abrasive or chemical cleaning, (b) post-welding processing of welded seams by cleaning flux and slag and by surface shaping to remove visible sharp projections and contours that identify concentrated stress areas, (c) surface treatment of the welded structure with corrosion resisting coatings, (d) thermal tempering for relaxation of residual stresses and for internally restructuring the metal grain in a manner reducing the influence of stress concentrations, and (e) demagnetizing treatment to protect welding arcs from magnetic interferences during multi-pass welding operations.
There are interactions of the various independent steps typically occurring at various times on metal products, particularly in view of various intricate work product shapes and loading patterns, and the difficulties in detecting defective subsurface base material patterns, such as grain structure and residual stresses in the product that affect fatigue, life and strength, particularly in the presence of stress concentration zones and highly loaded working zones. Thus, efforts in combatting long term fatigue initiated both during initial manufacture and during useful life with various technical operations heretofore available in the prior art have been substantially limited in their effectiveness and/or are unpredictable, thus producing compromised product quality inconsistent with expected and desired performance.
Known vibration and pulsed methods of stress relief include inducing low frequency mechanical vibrations into products such as welded structures to reduce residual stresses, and employing pulsed magnetic fields to relieve stress in ferromagnetic cutting tools.
At this stage of the prior art, a number and variety of interacting technical operations in a series of processing steps in initial production are required to fabricate proper welded metal products with greater load bearing capacity and lower internal residual stresses for longer expected life and higher quality. Simplification and lower cost of the production process as well as improved performance is thus highly desirable.
Welded metal product or structure manufacturing and repair practices require the addition of and/or removal of materials which therefore are consumed in the manufacturing process. For example, overlay welding and beading operations for strengthening weld seams require more initial product metal and require additional technical operations such as mechanical grinding, removal of fluxes and residues, thermal tempering and cosmetic shaping. It has not been feasible to obtain optimum appearance, strength and life in welded products without such steps. On the other hand, such steps increase costs of production and result in more complex fabrication process.
It is conventional to retire and replace aging metal structures such as steel bridgework and load bearing products subject to aging, which encounter stress fatigue corrosion, undesirable internal stress patterns, and the like, causing the presence of either unseen internal damage or observable surface defects. It is therefore desirable to provide improved maintenance and repair technology to extend the useful work product life by restoring or improving initial load bearing strength and reducing residual stresses in maintenance procedures so that current structures may be kept in operation.
In the welding structure arts conventionally in practice, practical technology has not been available which is well adapted for in-use non-destructive and non-deforming repairs to restructure and restore welded products that have become structurally unsound from aging, that have reduced loading capacity because of fatigue and residual stresses, or which have catastrophically failed by cracking, or the like.
For example, the prior art ability to repair visible catastrophic failures of structure, evidenced by cracks or fractures, in most part is limited to the addition of supporting braces, crutches, and other types of overlying structure to bypass damaged zones. Such techniques are not suitable for many metallic structure installations where there is either no accessible place to rework the welded products in-situ, where restrictions in space are imposed or where appearance of such bypassing structure is intolerable such as in bridgework and building structural support infrastructure.
One zone subject to residual stresses which may cause early failure is the junction zone between basic metal material and weld seams that may contain residual grain or stress patterns formed in the welding process. There are prior art techniques for annealing to redistribute and relax the stress patterns. However in general this is not a scientific method but an art dependent upon skills and experience of a few artisans, such as blacksmiths, where access to the work product is available. Such artistic methods have been applied for example in tempering knives or swords. One significant reason that such methods have not been replaced by scientific technology is that the nature of internal structure is difficult to ascertain and stress concentrations are of a diverse nature that defy analysis.
Thus, a serious deficiency with the manufacture and repair of structural and load bearing products is the lack of non-destructive detectors and corresponding automated systems that can both sense the nature of internal defects and correct them in diverse kinds of internal work product structure by restoring structural integrity to produce longer life following original manufacture procedures or renewed life imparted in maintenance procedures that overcome fatigue and internal stress patterns reducing product performance.
Accordingly, a specific objective of this invention is the introduction of novel procedures for sensing the nature of interior body grain and stress patterns, which is particularly important when involving metallic and ferromagnetic product lines either with or without welding seams.
Also detection of internal product structural conditions provides a frontier for novel automation procedures for radically improving the initial manufacturing phase of metallic or plastic bodies which are subjected to mechanical and thermal stresses in use.
Accordingly it is an objective of the present invention to correct such foregoing disadvantages of the prior art and to introduce production, maintenance and repair technology which can produce work products having idealized internal structure with improved load and wear capacity by elimination of residual stresses, voids and inferior grain structures that reduce product life.
Examples of typical prior art technology related to this invention or teaching some of the elementary underlying methodology now are briefly set forth, which in the present invention are interactively combined to produce novel combinations of technologies as a whole.
Overlay technology exists, such as filler welds and overlay welds, therein strengthening elements are superimposed over critical zones to bypass fatigued, fractured or other deficient welded product structure. The overlay may be superimposed directly upon weld seams in some cases. Typical examples of this technology are U.S. Pat. No. 2,537,533, G. E. Incalls, Jan. 9, 1951; RE 16,599 R. Mattice, Apr. 19, 1927; U.S. Pat. No. 1,703,111, S. J. Kniatt, Feb. 26, 1929; and U.S. Pat. No. 1,770,932, A. G. Leake, Jul. 22, 1930. Such overlay structure in U.S. Pat. No. 4,049,186 R. E. Hanneman, et al., Sep. 20, 1977 and U.S. Pat. No. 4,624,402, D. Pitcairn, et al., Nov. 25, 1986 in particular disclose overlay welds for the purpose of preventing stress corrosion failures in the welded body.
Peening by means of pellets, hammers, stress waves and ultrasonic impact is known to surface treat and deform the welded body surface structure for contouring weld sites to induce plastic deformation producing beneficial effects and heating of the metal for thermal tempering effects. Typical art of this nature includes U.S. Pat. No. 5,654,992, K. Uraki, et al., Aug. 5, 1997 and U.S. Pat. No. 3,961,739, B. P. Leftheris, Jun. 8, 1976. These disclosures recognize that mechanical pressure and stress waves applied to the external surface of a body creates thermal energy and a momentary state of plasticity in the workpiece.
In U.S. Pat. No. 4,330,699, M. M. Farrow, May 18, 1982, a non-contact laser welder is accompanied by a second amplitude modulated laser for generating acoustic waves in the melt to improve interdiffusion and homogeneity of the weld joint.
I have authored or co-authored several publications relating to ultrasonic impact treatment of welded joints and the relationship to fatigue resistance, typically as reported in the following International Institute of Welding IIW Documents:
Publication XIII-1617-96 for example discloses that the fatigue strength of as-welded joints was increased by changes in mechanical properties of material in surface layers induced by ultrasonic impact treatment (UIT). Thus, the material at the weld toe is compressed and deformed by manual indentation using an ultrasonic probe to form indented groove structure smooth and free from irregularities. This technique depends upon the training and skills of an operator manually wielding an ultrasonic probe to form the grooves, and requires reshaping of the weld site.
The comparison of peening with (UIT) is discussed in Document XIII-1668-97, which sets forth the advantages of ultrasonic impact treatment technology over peening, and the practicability of UIT technology to compress and indent the welded body structure in the vicinity of the weld seam.
The use of ultrasonic hand tools for achieving foregoing compression indentations is set forth in Document XIII-1609-95.
These techniques have demonstrated significant increases in fatigue limits of welded structures. However, this prior art technology requires physical distortion of the welded product or structure, and demands skilled labor to make decisions on the nature of indentations in the presence of different physical shapes of welded bodies and different loading requirements at the weld sites. Thus, it is neither practical nor economically feasible to apply the techniques universally or by automation to welded products of various sorts. Furthermore there can be no consistency from one product to another to assure constant quality performance expectations. Nor can techniques provided for initial welding production cycles only, be used for later maintenance of welded products or for repairs of cracks and other catastrophic failures.
The present invention has the objectives of curing deficiencies in the aforementioned type of prior art, and offering significant advantages in simplifying processing steps while guaranteeing higher quality products and improving useful life span and higher loading capacities of welded products at various stages of life, throughout the initial production of the product and even after catastrophic failures appear, such as visually observable cracks.
A significant objective of the invention is to coordinate and combine non-destructive ultrasonic impact treatment of work product bodies without deforming their shape in a procedure applicable to manufacturing, maintenance and repair processes, typically to relax internal stresses, reverse fatigue effects, improve corrosion fatigue strength and durability of load bearing surfaces and joints, and to create relaxed more ideally distributed internal body stress patterns.
It is a specific objective of the invention to introduce improved welding technology for improving product life spans, maintaining the products during useful life and repairing defects found in products to restore useful life.
Another objective of the invention is to reduce material consumption during welding while reducing the processing time and increasing the performance and life of welded products by replacing or eliminating various required technical operation steps required in the prior art in the production, maintenance and repair of welded products, such as grinding and surface shaping steps.
A further objective of the invention is to produce quality welded joints with consistently controlled stress distribution patterns, which may constitute either initially formed structure during manufacture or redistributed structure and stress patterns initiated during service life.
It is an objective of this invention to introduce life extension methods applicable to welded structural members to substantially increase useful lives and working strengths of the welded products in a manner not heretofore feasible.
It is a further object of the invention to develop scientific methods of treating bodies of metal, plastic and composite materials in a scientifically reproducible manner based upon detected dynamic internal body conditions exhibited during treatment procedures.